4° 



implements. 



following manner. In the first column the number of each 

 station is noted; in the second, the distance between the 

 stations in feet and inches ; in the third under foresights, 

 the heights in feet and tenths of feet in the direction of B 

 (see Figure 13); in the fourth, the height of the iustru- 



FIQ. 13.— LEVELLING. 



ment above the "datum-line," a line that can be fixed at 

 random, 10 or 100 feet below the surface; in the fifth, the 

 actual height above the datum-line ; in the sixth under back- 

 sights the height of the stations in the direction of A. 



It is necessary to note the "foresights" and "backsights" 

 only, in the field-notes, the rest is calculated afterwards as 

 follows. Supposing the datum-line to be fixed at 10 and 

 the backsight to station 1, is 7 feet, these sums added to- 

 gether give the height of the instrument at a, or 17 feet. 



